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cgroup: remove cgroup_pidlist->rwsem

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cgroup_pidlist locking is needlessly complicated.  It has outer
cgroup->pidlist_mutex to protect the list of pidlists associated with
a cgroup and then each pidlist has rwsem to synchronize updates and
reads.  Given that the only read access is from seq_file operations
which are always invoked back-to-back, the rwsem is a giant overkill.
All it does is adding unnecessary complexity.

This patch removes cgroup_pidlist->rwsem and protects all accesses to
pidlists belonging to a cgroup with cgroup->pidlist_mutex.
pidlist->rwsem locking is removed if it's nested inside
cgroup->pidlist_mutex; otherwise, it's replaced with
cgroup->pidlist_mutex locking.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
This commit is contained in:
Tejun Heo 2013-11-29 10:42:59 -05:00
parent e6b817103d
commit 069df3b7ae
1 changed files with 4 additions and 11 deletions
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15
kernel/cgroup.c
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@ -3462,8 +3462,6 @@ struct cgroup_pidlist {
struct list_head links;
/* pointer to the cgroup we belong to, for list removal purposes */
struct cgroup *owner;
/* protects the other fields */
struct rw_semaphore rwsem;
/* for delayed destruction */
struct delayed_work destroy_dwork;
};
@ -3522,7 +3520,6 @@ static void cgroup_pidlist_destroy_work_fn(struct work_struct *work)
struct cgroup_pidlist *tofree = NULL;
mutex_lock(&l->owner->pidlist_mutex);
down_write(&l->rwsem);
/*
* Destroy iff we didn't race with a new user or get queued again.
@ -3535,7 +3532,6 @@ static void cgroup_pidlist_destroy_work_fn(struct work_struct *work)
tofree = l;
}
up_write(&l->rwsem);
mutex_unlock(&l->owner->pidlist_mutex);
kfree(tofree);
}
@ -3612,7 +3608,6 @@ static struct cgroup_pidlist *cgroup_pidlist_find_create(struct cgroup *cgrp,
if (!l)
return l;
init_rwsem(&l->rwsem);
INIT_DELAYED_WORK(&l->destroy_dwork, cgroup_pidlist_destroy_work_fn);
l->key.type = type;
/* don't need task_nsproxy() if we're looking at ourself */
@ -3675,12 +3670,10 @@ static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
}
/* store array, freeing old if necessary */
down_write(&l->rwsem);
pidlist_free(l->list);
l->list = array;
l->length = length;
l->use_count++;
up_write(&l->rwsem);
mutex_unlock(&cgrp->pidlist_mutex);
@ -3762,7 +3755,7 @@ static void *cgroup_pidlist_start(struct seq_file *s, loff_t *pos)
int index = 0, pid = *pos;
int *iter;
down_read(&l->rwsem);
mutex_lock(&of->cgrp->pidlist_mutex);
if (pid) {
int end = l->length;
@ -3790,7 +3783,7 @@ static void cgroup_pidlist_stop(struct seq_file *s, void *v)
{
struct cgroup_pidlist_open_file *of = s->private;
up_read(&of->pidlist->rwsem);
mutex_unlock(&of->cgrp->pidlist_mutex);
}
static void *cgroup_pidlist_next(struct seq_file *s, void *v, loff_t *pos)
@ -3830,13 +3823,13 @@ static const struct seq_operations cgroup_pidlist_seq_operations = {
static void cgroup_release_pid_array(struct cgroup_pidlist *l)
{
down_write(&l->rwsem);
mutex_lock(&l->owner->pidlist_mutex);
BUG_ON(!l->use_count);
/* if the last user, arm the destroy work */
if (!--l->use_count)
mod_delayed_work(cgroup_pidlist_destroy_wq, &l->destroy_dwork,
CGROUP_PIDLIST_DESTROY_DELAY);
up_write(&l->rwsem);
mutex_unlock(&l->owner->pidlist_mutex);
}
static int cgroup_pidlist_release(struct inode *inode, struct file *file)